The present invention generally relates to the field of shaping light beams, in particular laser light beams. More specifically, the present invention relates to an optical system, an optical unit and to a method for shaping an incoming beam, in particular laser beam. Such an optical system, an optical unit and a method as mentioned before are in particular useful for producing a thin laser beam for material processing, for example for a directional crystallization of amorphous silicon films. Furthermore, the optical system, optical unit and the method according to the invention can be used in a solid state ring laser, for example.
Common lasers produce light beams which, on a macroscopic scale, appear to be exactly parallel with sharp edges when seen in cross-section of the beam. However, on a microscopic scale, there is an inherent divergence in the laser beam, i.e. the beam may be considered as a bundle of rays wherein the rays have slightly different propagation directions with respect to one another. The angle distribution of the laser beam intensity, thus, exhibits a profile which has a maximum in the main direction of propagation of the laser beam (angle 0°) and has a slope to both sides of the maximum (angle≠0°). Due to the natural divergence or angular spread of the laser beam, the edges of the laser beam are not exactly sharp but somewhat smeared out.
For many optical applications, in particular laser applications like annealing of semiconductors, a very low divergence of a beam is required.
Usually a field-stop is used to limit the field of view of an optical system (see for example Handbook of Optics, OSA, Eds. W. G. Driscoll and W. Vaughan, McGraw Hill, 1978, p. 2-52, W. J. Smith, Modern Optical Engineering, 3rd Ed., McGraw Hill, 2000, Ch. 6, p. 141-143). This approach is based on a spatial filtering, i.e. a diaphragm or a slit is used to reduce the size of the object which the system will image.
Specifically, U.S. Pat. No. 5,721,416 discloses an optical device for generating a sharp illuminating line on an illuminating plane from a high-power laser beam. This known device is based on spatial filtering. The sharp illuminating line includes long and short axes. The optical device comprises an anamorphic setup of imaging and homogenizing optical systems for the separate imaging and homogenizing of the laser beam in the direction of the long and short axes. For imaging and homogenizing the laser beam in the direction of the short axes, a slit is illuminated homogenously and the slit is imaged on the illumination plane by reducing optics. Thus, this known optical system also uses a slit for shaping the laser beam.
Despite the fact that an optical system using such a field-stop works quite well, the use of a slit or field-stop implies several drawbacks. One of the drawbacks arises when such a system is used in applications requiring a high energy density of the beam. Due to the high energy density of the beam, the body of the field-stop heats up to very high temperatures leading to deformations of the field-stop or slit. The result is that the beam shaping becomes inaccurate. Further, in order to produce very sharp edges of the light beam, the field-stop or slit must be manufactured with high precision machined sharp edges in order to be able to shape the incoming beam as desired.
U.S. Pat. No. 4,060,308 discloses an angle selective coupler for coupling optical energy into and/or out of optical fibers. The coupler consists of a section of the optical fiber modified in such a way as to allow optical excitation of a plurality of higher order modes of optical transmission, each mode being defined by a given angle of propagation relative to the fiber axis. In one embodiment the coupling section comprises a single strand of glass fiber waveguide which is tapered along its length. The existence of the tapered section allows coupling of radiation from an external source into a given propagation angle in the fiber. This document does not deal with the technical problem of producing a laser beam having sharp edges.
Due to the afore-mentioned drawbacks of the known optical systems and methods, there is still a need for an optical system and a method for shaping an incoming beam which does not rely on a spatial filtering.